The liquid collecting apparatus will be described taking a blood collecting apparatus which collects blood for example. The blood collecting apparatus is used for quantitative analyses in nuclear medicine diagnosis (e.g. PET (Positron Emission Tomography), SPECT (Single Photon Emission CT) and so on), and is used especially for measurement of a radioactive concentration in arterial blood of small animals (e.g. mice, rats and so on). Conventionally, the following modes (a)-(c) are employed in the quantitative analysis of the above small animals:
(a) Manual Blood Collection
Blood delivering itself under blood pressure from the other end of a catheter inserted into a mouse artery is received in a suitable receptacle. Then, a fixed volume of the blood in the receptacle is sucked up with a volumetric pipette, and a radioactive concentration in whole blood is measured by calculating (i.e. counting) radiation in the sucked-up blood. Further, plasma is obtained by centrifugal separation of the blood remaining in the receptacle, which is similarly collected with a volumetric pipette to measure a radioactive concentration in plasma.
(b) Artery Channel β-Ray Detector
A β+ ray detector is installed in an arterial blood channel to measure a radioactive concentration in blood. β+ rays are detected with a plastic scintillator or PIN diode. In Nonpatent Document 1, for example, a diode has a long and thin shape with a length of 30 [mm], and a detectable area is increased by installing a tube containing blood along the direction of a long side, thereby to secure detection efficiency.
(c) Microfluidic Device Mode
This is a mode which, as shown in FIG. 6, leads arterial blood delivering itself under mouse blood pressure onto a microchip (device) MC. The microchip MC has, arranged thereon, one main flow path FM, selectable branch flow paths FB, and side flow paths FN for feeding a mixed liquid H of physiological saline and heparin solution used for flow path cleaning or blood discharging, and for draining used mixed liquid H of physiological saline and heparin solution and blood B. A receptacle is placed at the end of each branch flow path FB, and one of the branch flow paths FB is selected by a gas pressure of argon gas Gas supplied to the microchip MC or a mechanism of the microchip MC. With one of the branch flow paths FB selected, blood B is poured in. The flow velocity of blood B and mixed liquid H of physiological saline and heparin solution is increased by placing the interior of each branch path of the microchip MC at negative pressure, and further by providing a peristaltic pump. Each of the flow paths FM and FB is formed by grooving the microchip MC in a predetermined size. It is the characteristic of the microchip MC that a minute volume of blood B is specified if a groove length or a groove area of the poured-in blood B is known. With the blood B of a predetermined volume filling the flow paths, based on the specified minute volume, the blood B is poured along with the mixed liquid H of physiological saline and heparin solution into a predetermined receptacle (not shown) by feeding the mixed liquid H of physiological saline and heparin solution under pressure. Subsequently, each of the flow paths FM and FB is cleaned with the mixed liquid H of physiological saline and heparin solution to be ready for a next blood collection. The blood B in the receptacle is washed out along with physiological saline into another receptacle, and the radiation in the blood B is counted with a well counter (see Nonpatent Documents 2 and 3, for example).
In order to reduce the amount of collected liquid and to secure the frequency of collection, there is a liquid collecting method for collecting a liquid to be measured, as separated in a time series, by inserting a gas or a liquid other than the liquid to be measured, as separators, at designated predetermined intervals into the liquid to be measured (see Patent Documents 1 and 2, for example). As a specific technique of obtaining plasma by centrifugal separation of blood as noted hereinbefore, there is a technique of separating blood into blood cell and plasma (that is, plasma separation), for example, by centrifugal separation in a U-shaped flow path (see Patent Document 3, for example).
[Nonpatent Document 1]    L. Convert, G. M. Brassard, J. Cadorette, D. Rouleau, E. Croteau, M. Archambault, R. Fontaine, and R. Lecomte, “A microvolumetric 3 blood counter for pharmacokinetic PET studies in small animals,” IEEE Nuclear Sci, vol. 54, no. 1, 2007.
[Nonpatent Document 2]    H.-M. Wu, G. Sui, C.-C. Lee, M. L. Prins, W. Ladno, H.-D. Lin, A. S. Yu, M. E. Phelps, and S.-C. Huang, “In vivo quantitation of glucose metabolism in mice using small-animal PET and a microfluidic device,” J Nucl Med, vol. 48, pp. 837-845, 2007.
[Nonpatent Document 3]    H.-M. Wu, R. W. Silverman, N. G. Harris, and R. L. Sutton, “Performing Longitudinal Measurements in Rodents Using Small Animal PET Imaging”, Conf Rec IEEE NSS & MIC, M10-398, 2008.
[Patent Document 1]    Unexamined Patent Publication S56-147013
[Patent Document 2]    Unexamined Patent Publication H1-307608
[Patent Document 3]    Unexamined Patent Publication No. 2004-109082